The objective of this project is to use the technological advancements in proteomic platforms to identify and validate clinically useful biomarkers found in human plasma that can be utilized in blood testing methods for breast cancer. Ideally, these screening methods will aid in the detection of cancer when it is in its earliest stages where treatment is more successful. Biomarkers of cancer could potentially be used for risk assessment, to detect existing disease, to decide on treatment options, and to measure cancer progression. Breast cancer is of significant public health importance with few available biomarkers and screening tests that are of the desired specificity. In an effort to identify biomarkers that could be used for the early detection of this cancer, I propose the use of an already conducted case-cohort collection within the Cardiovascular Health Study to evaluate the relationship between patterns of peptide expression of individuals that did or did not develop cancer. The proposed study will include a random sample of 14000 eligible subjects from the entire CHS cohort, these individuals will be compared to all eligible cases of breast cancer that are diagnosed in the entire cohort (-150). I will test baseline plasma obtained at the time of enrollment and plasma from the 4 year follow-up clinic visit of each subject and additional follow-up clinic visits. Plasma samples will be fractionated with acetonitrile to precipitate large globular proteins that make it difficult to evaluate the peptidome. The specific aims of this proposal are: 1. To obtain proteomic patterns of plasma samples collected pre-diagnosis from individuals with or without breast cancer using matrix-assisted laser desorption and ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) and compare these to discover biomarkers that may be useful for the early detection of this cancer;2. The proteomic techniques used in Aim 1 will be validated based on their ability to correctly predict cancer status using independent sample sets from individuals with and without breast cancer;and 3. To identify and characterize the peptides and proteins that were found to be cancer specific using various biochemical assays. Once identified these informative biomarkers will be evaluated for correlation with known cancer risk factors to see how combining risk factor data with proteomic data may enhance the utility of the informative proteins and peptides as early detection biomarkers. Furthermore, I will attempt to place the biomarker into context to test whether other proteins within a signaling or activation pathway are affected to understand the biology underlying the connection between the biomarker and breast cancer..